The present invention relates to processes for treating colored wastewaters from paper mills. More particularly, the present invention relates to processes for removing color from paper mill effluents and the destruction of the resultant precipitate.
A typical pulp mill is comprised of a variety of processes including wood chipping, cooking of the chips (the kraft sulfite process) to extract cellulose and hemicellulose (and to discard the lignin components), and bleaching to produce white pulp and/or paper. The pulp bleaching sequence commonly employs strong oxidants such as chlorine or chlorine dioxide which react with the lignin to make it water soluble. These components when mixed with the residual sulfite liquor from the pulping process produce a black, chemically undefined liquid which requires further wastewater treatment.
The bleach plant effluent stream (referred to herein as "E1"), commonly known as the E1 caustic sewer, generally has a high temperature (.apprxeq.70.degree. C.) and pH (.apprxeq.10.5). When softwoods are being treated, the effluent can be referred to as SE1. The effluent can be referred to as HE1 when hardwoods are being treated. The bleaching sequence also uses an acid wash which results in a second effluent stream, the acid sewer.
Typically, the acid and E1 sewers are mixed and then treated by conventional wastewater processes such as activated sludge, settling basins, secondary clarifiers, and other solids removal processes. These treatments decrease the carbon load (BOD.sub.5), remove phosphates and other eutrophying chemicals, but have no appreciable effect on color reduction.
An additional wastewater stream, the process sewer, is the second most concentrated source of color. This wastewater is comprised of spills and overflows from all other processes within the paper mill.
The color component in these wastewaters is refractory to the usual degradative processes. It results from a combination of conjugated ring structures (lignin) and the sulphates(ites) attached to the ring structures. Wastewater color exists in the form of colloidal particles (chromophores) with varying sizes and molecular weights. A smaller portion of the color is soluble in water and represents liberated, low molecular weight single ring structures. The chromophores and low molecular weight phenolics absorb ultraviolet light through conjugated double bond configurations, six carbon aromatic rings, nitrogen, sulphur and oxygen containing groups and heterocyclic compounds containing oxygen, nitrogen or sulphur. The colored effluent of a mill is composed of wood extractables and lignin degradation products formed during pulping and bleaching. A variety of condensation and oxidation reactions occur during cooking, and during chlorination or color extraction from pulp, producing quinoid structures. These structures are responsible for color absorbance in the visible spectrum. Since conventional wastewater treatment processes have no appreciable effect on color reduction, the color components remain in suspension/solution and are ultimately discarded into rivers or streams.
There are several problems associated with discharging colored wastewaters into the environment. First, they deteriorate the aesthetic appearance of the receiving streams and inhibit plant photosynthesis. This results in oxygen deficiency within the receiving stream which affects aquatic life forms.
Various processes have been developed or proposed to remove color from paper mill wastewaters. A number of these processes are discussed generally by V. R. Parthasarathy et al., "Decolorization of Pulp and Paper Mill Effluents." Int. Sem. Mgmt. Envir. Problems Pulp Paper Ind. (New Delhi) pp. 139-159, Feb. 24-25, 1982, and are examined in more detail below. These processes include (1) massive lime treatment of the effluent, (2) alum co-precipitation or precipitation with iron, salts and lime, (3) chemical oxidation using either potassium permanganate, oxygen, ozone or hydrogen peroxide, (4) adsorption/absorption on activated charcoal, (5) reverse osmosis/electrodialysis, (6) bio-genetics, (7) iron flotation or foam separation techniques, and (8) biological treatment.
In massive lime treatment, color imparted substances in the wastewater are deposited upon solid phase calcium hydroxide containing particles. These particles then need to be separated from the remaining wastewater and dewatered. The separation and dewatering steps are difficult to perform effectively and require a substantial amount of energy.
In the alum co-precipitation process, the alum (Al.sub.2 (SO.sub.4).sub.3) or iron salts (either FeCl.sub.3 or Fe.sub.2 (SO.sub.4).sub.3) act as both flocculating and precipitating agents. Although the use of alum for color removal is relatively cheap, the sludge is difficult to handle and cannot be disposed of directly as landfill. The iron salts are inefficient in removing color at low concentrations. However, increasing the concentration of these salts can actually increase the color level in the water because of the dissolved iron compounds. Further, while this technology is commonplace in the industry today, it is effective in only a narrow pH range (5.0-7.0). If the pH of the wastewater to be treated does not fall within this range, much of the alum is wasted in lowering the pH into the effective range which reduces the efficiency of the overall reaction. After solids settling, should the pH of the solution change, there exists the potential for particle resuspension.
In the chemical oxidation processes, oxidizing agents such as potassium permanganate, hydrogen peroxide, or ozone are added to the wastewater. The use of potassium permanganate is said to generate manganese dioxide which is brownish in color and water insoluble. The Parthasarathy article states "potassium permanganate has the lowest oxidation potential (0.59 mV) and generates MnO.sub.2 upon dissociation. KMnO.sub.4 is an active decolourization agent and it principally attacks the C.dbd.C and breaks it through oxidation. But, the ultimate product MnO.sub.2 is brownish colour and a water insoluble product. The finely dispersed brownish manganese dioxide has to be eliminated from the effluent by a final filtering process, which causes further complications." Although the Parthasarathy reference associates KMnO.sub.4 and active decolorization, it states that it produces a water insoluble product which complicates the filtration process, which appears to discredit the value of the reaction.
Hydrogen peroxide can also be added to the wastewater to remove color but requires a very long contact time for effective removal because the process relies on kinetic interaction between the peroxide and the chemical to be oxidized.
Ozone has also been studied as a possible oxidant for wastewaters and also relies on specific kinetic interactions which are primarily responsible for the cleavage of double bonds integral to aromatic ring structures. An example of a process that utilizes a combination of ozone and hydrogen peroxide for decolorizing lignin-containing aqueous solutions is disclosed in U.S. Pat. No. 5,190,669 to Weibel. One of the major problems associated with peroxide and ozone processes is the high expense necessary to employ the processes on a large scale.
In the adsorption and absorption processes, color removal is generally achieved through the use of either granular or powdered activated carbon. While this process has been shown to be effective in removing limited amounts of color, the quantity of activated carbon required for treatment of wastewaters from a paper mill is large and prohibitive to operate on a commercial scale.
The processes of reverse osmosis and electrodialysis have also been studied as potential techniques to remove color from spent liquors from pulp mill operations. Reverse osmosis has been primarily used in desalination of waters for the production of potable water and has been applied only minimally to other processes. Electrodialysis is complicated by the presence of multiple depolarizing agents inherent in the liquor which results in incomplete separation of compounds. To remedy this shortcoming, "ultrafiltration" has been used to completely separate compounds based on molecular weight. Though separation is more complete, the process is not commercially feasible due to the fragile nature of the membranes and the cost associated with the production of a filtration system capable of handling the large volumes associated with pulping processes.
The use of white rot fungus for the bio-genetic degradation of lignin has been studied. This technology involves expression of genes which code for ligninolytic enzymes. Wood chips are pretreated with fungal medium containing the secreted enzyme in hopes of reducing the amount of bleaching required. Fungal cultures have been shown to require additional carbon sources and in many cases demand nitrogen depleted environments to actively produce ligninase. Lignin degrading enzymes are produced during secondary metabolic processes and are present at low concentrations. Few established molecular biology protocols exist for the manipulation of fungal genes other than yeast. This area could be rapidly improved if the genes of the ligninolytic enzymes could be placed under the control of primary metabolic regulation to increase the level of production. Even if this were achieved, the method is limited by the rapid degradation of the enzymes at the temperatures used in the bleaching process.
Ion flotation or foam separation techniques have also been studied and have gained much attention in recent years. While the ion flotation technique appeared to be successful, its drawbacks are a very high installation cost as well as high operating costs.
Another process for removing color from wastewater is disclosed in U.S. Pat. No. 5,194,163 to Saugier. This patent discloses a process for decolorizing lignin containing aqueous solutions by the addition of peracetic acid or monoperoxysulfuric acid.
While many different processes have been proposed or experimented with, none has yet proven to provide an effective yet practicable solution to the problem of color removal. Accordingly, it would be a significant advancement in the art to provide a process for the removal of color from paper mill wastewaters which is both effective and economically feasible. It would be an even further advance if such a process could also produce a precipitate-free effluent. Such a process is disclosed and claimed herein.